The present invention relates to an electronic device provided with a cavity of which an internal pressure is held at a low level and a production method thereof. The present invention particularly relates to an electronic device in which a sensing unit of an infrared sensor or the like is hermetically closed in an atmosphere of a reduced pressure, and a production method thereof. In addition, the present invention relates to an electronic device in which a pressure of the atmosphere in such a cavity can be measured, and the pressure can be further reduced as required, and a production method thereof.
For the purpose of increasing detection sensitivity, in an electronic device such as an infrared sensor, conventionally, at least a sensing unit is disposed in a cavity formed on a substrate, and the sensing unit is hermitically closed in a vacuum atmosphere or an inert gas atmosphere by a cap unit.
Such electronic devices include, other than the infrared sensor, a pressure sensor, an acceleration sensor, a flow-rate sensor, a vacuum transistor, and the like.
Among such sensors, infrared sensors can be generally classified into thermal sensors such as a bolometer sensor, a pyroelectric sensor, a thermopile sensor, or a thermocouple sensor, and quantum-type sensors using PbS, InSb, HgCdTe, or the like. Many of the bolometer sensors comprise detecting units formed from a resistivity-changeable material such as polysilicon, Ti, TiON, or VOx, but some utilize a transient characteristic of a forward current of a PN diode, or the like. The thermopile sensor utilizes Seebeck effect caused in a PN junction, for example, and the pyroelectric infrared sensor utilizes the pyroelectric effect of a material such as PZT, BST, ZnO, or PbTiO3. The quantum sensor detects a current caused by electronic excitation. In addition, there is an infrared sensor having chromel-alumel thermocouple which detects infrared rays by Seebeck effect, or the like.
In order to maintain the detection sensitivity and the accuracy of the infrared sensor at high levels, it is preferred that heat dissipation from an infrared detecting unit be as small as possible. It is known that when the detecting unit is enclosed in a vacuum atmosphere or an inert gas atmosphere of a reduced pressure which is hermetically sealed by a micro vacuum package, or the like, the detection characteristics are improved.
The sensitivity of the pressure sensor or the acceleration sensor is also improved when the viscous resistance of the air existing around the detecting unit is lowered, so that it is preferred that the detecting unit be enclosed in a vacuum atmosphere or an inert gas atmosphere of a reduced pressure which is hermetically sealed by a cap unit, or the like. When the interior of the cap unit is sealed so as to be in a vacuum condition, preferably, it can be confirmed that the vacuum level in the cap unit can be held in the production or in the use of the electronic device.
Hereinafter with reference to FIGS. 1A to 1F, a conventional method of fabricating an electronic device will be described.
In a step shown in FIG. 1A, a silicon substrate 101 on which a sensing unit 102 of an infrared sensor or the like is formed is prepared. After a silicon oxide film 103 is deposited on the substrate by CVD, for example, the silicon oxide film 103 is patterned so as to cover the sensing unit 102 and the peripheral portion thereof. Since the silicon oxide film 103 functions as a sacrificial layer, the silicon oxide film is removed by etching in a succeeding step, thereby defining the shape of a cavity.
In a step shown in FIG. 1B, a polysilicon film 104 is formed so as to cover the silicon oxide film 103 by CVD. The polysilicon film 104 constitutes a sidewall and a ceiling wall of a cap unit of the electronic device.
In a step shown in FIG. 1C, a number of etching holes 111 which run through the polysilicon film 104 and reach the silicon oxide film 103 are formed.
In a step shown in FIG. 1D, hydrofluoric acid is injected through the etching holes 111, so as to dissolve the silicon oxide film 103. The solution is removed via the etching holes 111. As a result, a cavity 112 surrounded by the silicon oxide film 103 is formed, and the sensing unit 102 of the sensor is exposed in the cavity 112.
Next, in a step shown in FIG. 1E, a polysilicon film 106 is deposed by CVD so as to cover the polysilicon film 104. At this time, the polysilicon film 106 is also deposited in interior walls of the etching holes 111, so as to close the etching holes 111. In a time period after the start of CVD until the etching holes are completely closed, the polysilicon film 106 is deposited on an interior wall of the cavity 112.
The above-mentioned CVD process is generally performed by using a reaction gas such as SiH4 under a pressure of 500 mTorr (about 67 Pa). Therefore, the cavity 112 is hermetically sealed in a condition where the internal pressure is about 500 mTorr (about 67 Pa) in the CVD process. In addition, in the CVD process, SiH4 which is not yet reacted and an H2 gas caused by the reaction remain in the interior of the cavity 112. Moreover, SiH4 which is not yet reacted and the H2 gas caused by the reaction are adsorbed to the polysilicon film 106 deposited on the wall of the cavity 112.
Next, in a step shown in FIG. 1F, the whole of the substrate 101 is heated at high temperatures of 500° C. or more under high vacuum. At this time, the SiH4 gas in the cavity 112 is decomposed to some extent, and the H2 gas is discharged to the exterior through the polysilicon films 104 and 106. Accordingly, the pressure in the cavity 112 is slightly reduced from the internal pressure of the cavity 112 in the CVD process, so that the vacuum level of the cavity 112 is somewhat increased.
The above-described production method is described in Japanese Laid-Open Patent Publication No. 2000-124469, for example.
Next, a prior art for increasing the vacuum level in the interior of a vacuum package (a cap unit), and a prior art for measuring a vacuum level (a pressure) will be described.
FIG. 42 schematically shows a sectional configuration of an electronic device having a conventional vacuum package (see Japanese Laid-Open Patent Publication No. 11-326037). The electronic device shown in FIG. 42 includes a silicon substrate 91, a transmitting window 94 fixed on the silicon substrate 91 by means of a solder 99. A gap 93 having a height of about 1 to 10 mm is disposed between the transmitting window 94 and the silicon substrate 91. A getter 95 having a size of several millimeters is disposed in the gap 93.
A through hole 97 is formed in the transmitting window 94, and the getter 95 is disposed in the gap 93 through the through hole 97. When the silicon substrate 91 is disposed in a vacuum, the gap 93 is evacuated through the through hole 97, so that the pressure is reduced. The through hole 97 is closed by melting the solder 99 for vacuum sealing, so as to hold the gap 93 in a vacuum condition. Thereafter, when the getter 95 is activated, the pressure of the gap 93 is further reduced, and a high vacuum condition can be attained.
The vacuum level in the cap unit can be measured by using a Pirani gauge, for example. The Pirani gauge is an apparatus for obtaining a vacuum level based on an electric resistance value of the resistive element disposed in a vacuum. The coefficient of thermal conductivity of a gas depends on a pressure of the gas, i.e., a vacuum level. For this reason, if the coefficient of thermal conductivity from a heated resistive element to the gas is obtained, it is possible to determine the vacuum level of the gas by appropriate calibration.
Recently, electronic devices are miniaturized, so that the above-described vacuum packages (cap units) are more strongly required to be microminiaturized. For example, an image sensor in which a number of infrared detecting units and visible-light detecting units arranged in a matrix are provided on one and the same substrate is proposed. In such an image sensor, each of the infrared detecting units having a size of about 50 μm×50 μm is sealed by a micro vacuum package having a size of about 100 μm×100 μm (Japanese Laid-Open Patent Publication No. 2003-17672).
In order to manufacture a microminiaturized electronic device on which an FEA device and a transistor for performing high-speed switching operation in a vacuum are mixedly mounted, a technique in which a microminiaturized vacuum package is formed only in a portion of the FEA device on the substrate is described, for example, in Silicon metal-oxide-semiconductor field effect transistor/field emission array fabricated using chemical mechanical polishing, C. Y. Hong and A. I. Akinwande, J. Vac. Sci. Technol. B Vol. 21, No. 1, p 500-505, January/February 2003.
According to the above-described method of fabricating the electronic device, the SiH4 gas is decomposed in the cavity 112 in the thermal treatment step shown in FIG. 1F, and the H2 gas is discharged to the exterior of the cavity 112. Therefore, the vacuum level in the cavity is slightly increased as compared with the pressure of 500 mTorr (about 67 Pa) in the CVD process. However, there exists a problem that an increase in vacuum level is not expected, for the purpose of improving the sensitivity of the sensor.
In the above-described production method, any cavity is not formed between the detecting unit 102 and the substrate 101. By disposing sacrificial layers for respective upper and lower layers of the detecting unit 102, it is possible to fabricate a configuration in which the atmospheric gas in the cavity is in contact not only above but also below the detecting unit 102.
FIG. 2 is a perspective view showing the vicinity of a detecting unit of a bolometer-type infrared sensor having such a configuration. In FIG. 2, a resistive element 151 referred to as a “bolometer” functioning as an infrared detecting unit, and a supporting member 152 for supporting the resistive element 151 are formed on a substrate 101. The resistive element 151 is formed from a patterned polysilicon film, for example. The supporting member 152 is often disposed by laminating a polysilicon film, a nitride film, an oxide film, and the like. The supporting member 152 has arm portions extended from a supporting main portion on an upper face of which the resistive element 151 is formed. The supporting member 152 is fixed to the substrate 101 via the arm portions.
In FIG. 2, a cavity-wall member is not shown. In an actual infrared sensor, a supporting member 150 is disposed in an interior of a cavity which is the same as the cavity 112 shown in FIG. 1F.
Hereinafter, a problem caused in the case where the etching holes are closed by CVD will be described in detail.
Although not shown in FIG. 2, when infrared rays pass through the polysilicon films surrounding the cavity (the films indicated by the reference numerals 104 and 106 in FIG. 1F) and are incident on the resistive element 151, the temperature of the resistive element 151 is increased. In conjunction with the temperature rise, the resistance value is varied. The infrared sensor having the configuration of FIG. 2 measures the change of the resistance value, so as to detect the amount of infrared rays incident on the resistive element 151.
In order to increase the detection sensitivity of the infrared sensor, it is necessary to increase the magnitude of temperature rise of the resistive element 151 when the infrared rays are incident on the resistive element 151. Therefore, it is preferred that the resistive element 151 functioning as an infrared detecting unit and the exterior thereof be thermally insulated as much as possible.
The thermal conductance between the resistive element 151 and the exterior thereof is classified into thermal conductance via the supporting member 152 which connects the resistive element 151 to the substrate 101, and thermal conductance via a gas around the resistive element 151.
The thermal conductance via the supporting member 152 becomes smaller, as a sectional area of the narrowest portion of the supporting member 152 is smaller, and as the distance from the substrate 101 is larger. For example, if a technique of MEMS (Micro-Electro-Mechanical Systems) is used, it is possible to configure the portion (the connecting portion) of the supporting member 152 coupled to the substrate 101 by two columns of Si3N4 having a sectional area of 3 μm2 and a length of 50 μm. In this case, the thermal conductance is 3×10−7 (W/K).
On the other hand, the thermal conductance via the gas around the resistive element 151 is smaller, as the pressure of the gas is smaller. For this reason, it is necessary to reduce the pressure of the gas around the detecting unit, in order to increase the sensitivity of the infrared sensor.
However, in the conventional production method described with reference to FIGS. 1A to 1F, after the step shown in FIG. 1E, the internal pressure of the cavity 112 is maintained at about 500 mTorr (about 67 Pa) by the residual gas. After the formation of the cavity 112, vacuum and high temperature treatment is performed, thereby diffusing hydrogen in the interior into the exterior. Thus, the internal pressure of the cavity 112 can be somewhat reduced, but the SiH4 gas which cannot be discharged to the exterior of the cavity 112 by the high temperature heating remains in the cavity.
In the infrared image sensor such as a bolometer type, a relationship shown in the graph of FIG. 3 exists between the pressure of the gas covering the detecting unit and the sensitivity. Such a relationship is described, for example, in “Uncooled Infrared Imaging Arrays and Systems” by Academic Press, page 115.
In the graph of FIG. 3, the ordinate indicates the sensitivity, and the abscissa indicates the atmospheric pressure of the detecting unit 12. As is seen from the graph, as the pressure is lower, the sensitivity increases. The sensitivity in the case of the pressure of 50 mTorr is about three times as much as the sensitivity in the case of the pressure of 500 mTorr. Therefore, it is desired that the internal pressure of the cavity is 50 mTorr or less.
The supporting member 152 for the detecting unit 151 of the infrared sensor has a minute configuration as shown in FIG. 2. Thus, if the heating at extremely high temperatures is performed in the step of FIG. 1F, thermal stress is generated in the supporting member 152, so that the supporting member 152 may be broken.
In the case where high temperature heating of 660° C. or more is performed, there arises a problem that Al used as wiring for the sensor is molten. Thus, it is necessary to perform the heating at the temperature or lower temperatures. However, in such temperatures, the diffusion velocity of H2 to the exterior is very low, so that the function as the heating for increasing the vacuum level is not expected so much.
As described above, by the conventional production method in which the etching holes are closed by CVD, it is difficult that the vacuum level of the cavity 112 is further increased, and hence the detection sensitivity is increased.
If the method described with reference to FIG. 12 is adopted in order to increase the vacuum level, it is extremely difficult to dispose the getter shown in FIG. 12 in the minute cavity with good yield.
If the above-mentioned vacuum package (the cap unit) is miniaturized so as to have a size of 1 mm or less, it becomes further difficult to dispose the getter agent in the interior of each vacuum package by a conventional method. For example, in the case where each of the infrared detecting units is sealed by a micro vacuum package having a size of about 100 μm×100 μm, it is very difficult and it takes a lot of trouble to disposed a getter agent in the interior of each of a number of vacuum packages.
Moreover, many of the techniques for detecting the vacuum level by a conventional Pirani gauge are produced for the purpose of measuring the vacuum level in a vacuum chamber of a large-size apparatus. Thus, the smallest detecting device has a length of about 0.2 inches. Therefore, the conventional Pirani gauge is not suitable for measuring the internal pressure of the above-mentioned micro vacuum package.